Well, it's not about size really. It can be about visible range. Most stock turnsignals can be seen from the front and rear, and from the side.
Tne signals in your link look as though they would be unable to be seen directly from the side. This could be bad, and may be the reason they are not DOT approved. I wouldn't use them, but keep in mind laws, and most importantantly, enforcement , varies alot from place to place. Not sure a cop will pull you over because of them, or if you have safety inspections where you live, if they'd notice anything.
Your choice here on taking that chance. But I'd look for some with DOT approval, or at least better visible range.